1. Field of the Invention
The invention relates to the production of high purity silane and silicon. More particularly, it relates to an improved process for enhancing the production of said desired products.
2. Description of the Prior Art
The development of new techniques for the utilization of non-polluting sources of energy is of paramount national and world-wide interest. Solar energy is among the energy sources of greatest interest because of its non-polluting nature and its abundant, non-diminishing availability. One approach to the utilization of solar energy involves the conversion of solar energy into electricity by means of the photovoltaic effect upon the absorption of sunlight by solar cells.
Silicon solar cells, the most commonly employed devices based on the photovoltaic effect, have been employed reliably in space craft applications for many years. For such applications and for industrial and commercial applications in general, crystals of high purity, semiconductor grade silicon are commonly employed. Such high purity, high perfection silicon is generally produced by procedures involving converting metallurgical grade silicon to volatile trichlorosilane, which is then refined by the chemical and physical methods and finally reduced to produce polycrystalline, semiconductor grade silicon from which single crystals can be grown. The costs associated with the production of such high purity, high perfection crystals are high.
The initial step of converting metallurgical silicon to trichlorosilane has commonly been carried out by reacting metallurgical grade silicon with HCl in a fluid bed reaction zone at about 300.degree. C. Trichlorosilane comprises about 85% of the resulting reaction mixture, which also contains silicon tetrachloride, dichlorosilane, polysilanes and metal halides. While this technique has been employed successfully in commercial practice, it requires the use of relatively large reaction vessels and the consumption of excess quantities of metallurgical silicon. In addition, the reaction mixture is relatively difficult to handle and has associated waste disposal problems that contribute to the cost of the overall operation.
In producing high purity polycrystalline silicon from trichlorosilane, current commercial technology is a low volume, batch operation generally referred to as the Siemens process. This technology is carried out in the controlled atmosphere of a quartz bell jar reactor that contains silicon rods electrically heated to about 1100.degree. C. The chlorosilane, in concentrations of less than 10% in hydrogen, is fed to the reactor under conditions of gas flow rate, composition, silicon rod temperature and bell jar temperature adjusted so as to promote the heterogeneous decomposition of the chlorosilane on the substrate rod surfaces. A general description of the Siemens type process can be found in the Dietze et al patent, U.S. Pat. No. 3,979,490.
Polycrystalline semiconductor grade silicon made from metallurgical grade silicon costing about $0.50/lb. will, as a result of the cost of such processing, presently cost on the order of about $30/lb. and up. In growing a single crystal from this semiconductor grade material, the ends of the single crystal ingot are cut off, and the ingot is sawed, etched and polished to produce polished wafers for solar cell application, with mechanical breakage and electronic imperfection reducing the amount of useable material obtained. As a result, less than 20% of the original polycrystalline, semiconductor grade silicon will generally be recoverable in the form of useable wafers of single crystal material. The overall cost of such useable material is, accordingly, presently on the order of about $300/lb. and up. Because of the relatively large area requirements involved in solar cell applications, such material costs are a significant factor in the overall economics of such applications.
The economic feasibility of utilizing solar cell technology for significant portions of the present and prospective needs for replenishable, non-polluting energy sources would be enhanced, therefore, if the overall cost of high purity single crystal wafers could be reduced. One area of interest, in this regard, relates to the development of a low-cost, continuous process for the production of polycrystalline silicon from silane or chlorosilanes. The decomposition of such silanes in a fluid bed reaction zone is disclosed in Ling, U.S. Pat. Nos. 3,012,861 and Bertrand, 3,012,862. Another approach for the continuous production of silicon from silane is that disclosed in German Patent Specification Nos. 752,280, published May 26, 1953, and 1,180,346, published July 1, 1965. In this approach, the silane is heated to above its decomposition temperature quickly in a nozzle and is then caused to expand into a substantially cooler chamber from the bottom of which silicon product is collected. A second area of interest of the development of lower cost has been the production of silane by the disproportionation of trichlorosilane. One suggested approach involves the use of a bed of insoluble, solid anion exchange resin in a distillation system from which silane is recovered at the top and from which SiCl.sub.4 is withdrawn at the bottom as disclosed in Bakay, U.S. Pat. No. 3,968,199. Another area of interest resides, of course, in the initial production of trichlorosilane from the metallurgical grade silicon. Improved processing permitting a reduction in the number or size of the reaction vessels employed, or simplifying the handling of the reaction mixture and reducing the waste disposal problems involved, would contribute significantly to the overall reduction in the cost of high purity silane and/or silicon. Such reduction in costs through simplified processing operations is desired in the art not only in the field of solar cell technology, but to enhance the prospects for the use of such high purity silicon for semiconductor applications as well. In addition to such specific areas of interest for possible processing improvement, a genuine need exists for integrated overall processing improvements to reduce overall costs, simplify feed material requirements and reduce waste disposal and other material disposal considerations. For example, the Bakay process referred to above produces by-product SiCl.sub.4 which must be utilized, sold or otherwise disposed of in the overall processing operation. The commonly employed process for producing trichlorosilane, on the other hand, requires a source of HCl, adding to the cost and the material handling requirements of the process. An integrated process for the conversion of metallurgical grade silicon to high purity silane and silicon, with simplified material requirements and reduced waste disposal, is genuinely needed in the art, therefore, to enhance the prospects for effectively utilizing high purity silicon on a commercially practical basis for solar cell and semiconductor applications.
It is an object of the present invention, therefore, to provide an improved process for the production of high purity silane.
It is another object of the invention to provide an improved process for the conversion of metallurgical grade silicon to high purity silane and high purity silicon.
It is another object to provide an integreted process for the production of high purity silane and high purity silicon with simplified feed material and reduced material disposal requirements.
It is a further object of the invention to provide a process for the production of silane from metallurgical grade silicon incorporating an enhanced process for the initial production of trichlorosilane from metallurgical grade silicon.
With these and other objects in mind, the invention is hereinafter disclosed in detail, the novel features thereof being particularly pointed out in the appended claims.